Advanced Friction Stir Spot Welding of Aluminum Alloy to Transformation Induced Plasticity Steel

Abstract

In recent years, growing amount of attention has been drawn to studies of reducing vehicle weights in automotive industry considering both economic and environmental factors. One of the effective methods to achieve this goal is to replace steel components with multi-material vehicle structures. Joining of dissimilar materials is therefore very important to meet this demand. It is generally difficult to join dissimilar materials with the conventional welding technologies, as they have quite different physical and chemical properties. In addition, the formation of large amount of brittle intermetallic compounds (IMCs) is highly detrimental to joint quality. Friction stir welding (FSW) is a solid state welding technique and can avoid bulk melting during the process. As a variant of friction stir welding, friction stir spot welding (FSSW) is a promising solution to make spot joints between dissimilar materials. However, limited studies have been conducted on the FSSW of dissimilar materials, specifically aluminum alloy to steel. When performing the FSSW, the keyhole left on the welding zone deteriorates the joint strength and the process requires a large plunge force to make a spot welding with the dissimilar materials. To solve the existing issues, the FSSW process has been successfully applied to join aluminum alloy 6061-T6 to TRIP 780 steel. Cross sections of weld specimens show the formation of a hook with a swirling structure. A higher magnified SEM view of the swirling structure with EDS analysis reveals that it is composed of alternating thin layers of steel and Al/Fe intermetallic compounds (IMCs). To evaluate the effects of different process parameters including tool plunge speed and dwell time on the weld strength, the design of experiments (DOE) and analysis of variance (ANOVA) is used. It shows that dwell time is a more dominant parameter in affecting the weld strength than plunge speed. Furthermore, the investigation of failure after applying lap shear tests reveals that the cross nugget failure is the only failure mode. A FSSW process model is developed in this study based on the Coupled Eulerian-Lagrangian (CEL) method, which considers the material flow at the dissimilar material interface and the interaction between the welding tool and workpiece. The force and thermal history generated by the numerical work also correspond well with the experimental data. In the experimental investigation of applying the proposed keyhole refilled FSSW process to join aluminum alloy 6061-T6 to TRIP 780 steel, it is shown that the original keyhole area is filled with the aluminum alloy and no obvious voids can be found at the Al/Fe interface. The lap shear force produced by the keyhole refilled FSSW increased by 55.98% when compared to the conventional FSSW. Three bonding mechanisms can be concluded according to the experimental observations: (1) the keyhole is refilled by the aluminum alloy; (2) the bonding area is increased between the steel and aluminum; and (3) the hooks developed from the conventional FSSW process generate bonding between the steel and aluminum. Finally, the electrically assisted FSSW process for joining aluminum alloy 6061-T6 to TRIP 780 steel is performed. The experimental results show that the application of electrical current reduces the axial plunge force and assists the material flow of the aluminum matrix during the welding process.